1. Field of the Invention
The present invention relates an optical pickup apparatus. More particularly, the present invention relates to an optical pickup apparatus which is capable of recording/reproducing information on/from recording media having different formats by employing a dual wavelength laser diode and a hologram element.
2. Related Art
An optical pickup apparatus is commonly employed in an optical recording/reproducing apparatus, such as a compact disk (CD) player, a digital versatile disk (DVD) player, a CD-ROM driver, and a DVD-ROM to record/reproduce data on/from a recording medium, such as an optical disk, in a non-contact manner. In a recording mode, an optical pickup projects laser beams onto a surface of an optical disk to thereby form a pit. In a reproducing mode, the optical pickup optically reads out pit information from the optical disk and outputs read information in an electric signal form. In order to achieve the above performance, the optical pickup comprises a plurality of optical members, such as a laser diode for emitting laser beams, a diffraction grating, a beam splitter for adjusting deflection of the laser beam, a plurality of lenses for forming a light path, and a sensor for detecting a signal.
If the optical pickup apparatus is employed in the DVD player that is capable of high-density recording/reproduction, it has to be compatible with a compact disk (CD) or a CD-Recordable (CD-R).
However, the DVDs have a standard thickness that is different from that of the CDs. This is because of an allowable error of a mechanical disk slope and numerical aperture of an object lens. The thickness of CD is 1.2 mm, whereas the thickness of DVD is 0.6 mm. Regarding a wavelength of a light source for the reproduction, the CD is standardized to 780 nm, whereas the DVD is standardized to 650 nm. Due to the above differences, a general CD player is not able to reproduce data recorded on a DVD. Therefore, there is a demand for an optical pickup apparatus for DVD that is compatible with a general CD.
FIG. 1 illustrates a conventional optical pickup apparatus which has been developed in order to solve the above problem. Referring to FIG. 1, the conventional optical pickup apparatus comprises a light module 10 having a first light source 11 and a monitor photo-detector 13 which are integrally formed therewith; a second light source 20; a photo-detector 21 arranged to adjust an amount of light rays emitted from the second light source 20 in a data recording mode; a ½-wavelength plate 25 arranged to convert a p-polarized light emitted from the second light source 20 into a s-polarized light ray, a complex prism 30 acting as a polarized light beam splitter to convert light paths of light rays respectively emitted from the first and the second light sources 1, 20; a collimating lens 40 arranged to convert the light rays emitted from the first and the second light sources 11, 20 into parallel light rays; a reflection mirror 45 arranged to reflect incident light rays; a polarized light hologram element 50 arranged to split a light which is incident to a disk 100 to a plurality of beams; an object lens 60 for focusing the split beams on the disk 100, and a third photo-detector 80 for receiving light beams which are reflected from the disk 100 and transmitted through the complex prism 30. One of the first and the second light sources is operated. The first light source 11 is configured for recording/reproducing data on/from a CD, and emits light rays of 780 nm wavelength. In contrast to the first light source 11, the second light source 20 is configured for recording/reproducing data on/from a DVD, and emits light rays of 650 nm wavelength.
The polarized light hologram element 50 is designed to operate in response to the wavelength of light rays emitted from the second light source 20, i.e., in a DVD mode. In addition, the polarized light hologram element 50 operates only in response to the p-polarized light. The polarized light hologram element 50 has a ¼-wavelength plate formed on a surface thereof opposing to the disk 100 so that the polarized light hologram element 50 can convert an incident s-polarized light into a circularly polarized light and convert a reflection light from the disk 100 to the p-polarized light. Only the light containing a p-polarized light, which is emitted from the second light source 20 and reflected from the disk 100, is split into 10 beams by the polarized light hologram element 50.
In addition, a third photo-detector 80 is further arranged to receive the 10 split beams to thereby obtain data information and error information from the disk 100. The third photo-detector 80 comprises a plurality of sensors for detecting light rays reflected from the DVD and a single sensor for detecting light rays reflected from the CD.
Since the conventional optical pickup apparatus as described in connection with FIG. 1, aligns optical axes of the lights having different wavelengths by use of the complex prism 30 and achieves a simple optical structure by use of the collimating lens 40, an improved signal regeneration can be obtained when operating at a high temperature (recording and reproducing). However, it is difficult to fabricate the complex prism 30, as a variety of technologies are required in assembling the complex prism 30 with such an optical pickup apparatus. In addition, an assembly tolerance between the object lens 60 and the polarized light hologram element 50 cannot be solved, and, similarly, the pickup adjustment cannot be achieved due to the splitting of light into 10 beams. Moreover, since two laser diodes are used, an optical axis error occurs between the two lights emitted from the two laser diodes. Due to the complicate structure and a great number of assembly parts required, the assembly process deteriorates, which causes an inferiority of a product.
Since a power of a laser in a DVD recording mode is stronger that that in a reproducing mode, the optical pickup apparatus is operated at a relatively high temperature. Since inner parts of a pickup head are assembled using an UV (Ultraviolet) bond, connection portions may warp and expand at the high temperature. In this case, the light transmitted through the parts or reflected from the parts deviates from the photo-detector, which deteriorates the signal regeneration at the high temperature. In order to solve the above problems, there is a demand for new ways to remove unnecessary parts and/or minimize the number of beams deviating from the photo-detector.
Furthermore, the conventional optical pickup has problems of a lowered productivity which is caused by difficulty in assembling, a reduction of yield, and an increased manufacturing cost which is caused by the great number of optical elements.
Accordingly, there is a need for an optical pickup apparatus for use in an optical recording/reproducing apparatus to record/reproduce data on/from recording media having different formats that is simple in terms of construction and assembly, and that is effective in terms of manufacturing cost, adjustment and signal regeneration while operating at a high temperature.